In wireless radio frequency telecommunications, the MIMO technology (multiple input multiple output) relates to the use of multiple send and receive antennas for a wireless communication. The MIMO technology forms the basis for coding methods which use the temporal as well as the spatial dimension for transmitting information (space/time coding). Thus, a quality and data rate of a wireless connection may be increased.
A typical so-called massive MIMO system comprises a plurality of user equipments arranged within a cell served by a base station having a plurality of antennas. In a massive MIMO system location information of each of the user equipments as such may not be useful to configure the individual antenna transceivers of the base station, especially in a rich scattered environment. A rich scattered environment relates to for example street canyons of a city where a direct line of sight (LOS) between the base station and the corresponding user equipments can not be reached most of the time and radio signals between the base station and the user equipments may be reflected several times at buildings and other obstructions. Hence, the phase and the amplitude that needs to be set to the individual transceiver elements can not be calculated from the location information. Therefore, configuration of the individual transceiver elements may be based on test signals or training sequences transmitted from one user equipment while the other user equipments within the cell are silent or signals from the other user equipments are separated from the training signals by for example an orthogonal coding like CDMA. However, when a user equipment is moving, the configuration derived from the training signal may become obsolete very soon as the focus of the typical massive MIMO system may be about some tenth of the wavelength only, especially if the user equipment is arranged in a rich environment and/or has no direct line of sight to the base station. The focus can be thought of as a fading dip, but with constructive interference. When the user equipment moves out of the focus, a new configuration is needed. Therefore, the user equipment needs to resend a training signal for the antenna transceiver array at the base station to calibrate on. Hence, the frequency or interval of which the recalibrations are done determines the maximum velocity for a user equipment in such a system.
FIG. 1 shows as an example of estimated angles of arrival of radio signals received from three different user equipments in a MIMO base station having a linear antenna array of twenty antennas in a rich scattered environment. The estimated angles of arrival for the first user equipment are indicated by the symbol “x”, the angles of arrival of the second user equipment are indicated by the symbol “+” and the angles of arrival of the third user equipment are indicated by the symbol “*”. As can be seen, the third user equipment is located at an angle of approximately 160° and has a rather low deviation in the angle of arrival, for example because the third user equipment is arranged such that it has a direct line of sight to the base station. However, the angels of arrival of the second user equipment are more spread, having a focus near 120°. The first user equipment, which may be arranged for example at an angle of 180°, has angles of arrival spread all over the range from 0 to 180° for example due to strong scattering in the environment.
As can be seen from the above, the transceiver elements of the base station have to be reconfigured or recalibrated whenever the user equipment is moving. This requires a more frequent transmission of training signals which may reduce the overall transmission capacity within the cell.
Therefore, there is a need for an improved reconfiguration of the transceiver elements in a massive “MIMO” with moving user equipments.